1. Field of the Invention
The present invention relates to a gear shift control system for an automatic transmission for an automobile and, more specifically, to an automatic transmission gear shift control system which varies an upward and downward change in the locking pressure applied to frictional coupling elements according to driving conditions during gear shifting.
2. Description of Related Art
Typically, automatic transmissions for automobiles combine a torque converter and a transmission gear mechanism provided with a plurality of frictional coupling elements, such as brakes and clutches, which are selectively locked and unlocked to automatically switch the torque transmission paths according to driving conditions in order to provide desired gears. Locking and unlocking of these frictional coupling elements is controlled by a hydraulic control circuit. This hydraulic control circuit includes a regulator valve for regulating hydraulic line pressure discharged from a hydraulic pump to a specified pressure level, a manually operated shift valve for placing the automatic transmission into a selected one of available ranges of the automatic transmission, and a plurality of shift valves for selectively actuating pressure lines in communication with actuators for the frictional coupling elements. Further, a duty solenoid valve may be installed in the hydraulic circuit so as to vary the regulated level of line pressure according to driving conditions defined by, for instance, engine throttle openings.
In order for such an automatic transmission to alleviate shift shocks, the line pressure or locking pressure applied to selected frictional coupling elements is varied such that it rises via what is called a "transitional level of pressure" which is between an initial pressure level and a target pressure level and at which the line pressure is intentionally maintained for a while. For this purpose, the hydraulic control circuit includes accumulators located between the shift valve and pressure lines in communication with actuators for selected frictional coupling elements for developing a desired level or state of transitional line pressure. Such an automatic transmission control system is known from, for instance, Japanese Patent Publication No. 63-3183. In a case in which the level of transitional line pressure is too high during gear shifting, a very short shifting time is required for the automatic transmission to complete gear shifting and consequently, enhanced shift shocks occur. Contradistinctively, in a case in which the level of transitional line pressure is too low during gear shifting, the frictional coupling elements tend to slip unnecessarily and prolong a shifting time so as not only to deteriorate the feeling of gear shifting but also to lower the durability of operation thereof. For this reason, the transitional level of pressure is desirably set so as to complete gear shifting of the automatic transmission in a short time without being accompanied by immoderate shift shocks. However, in spite of keeping the transitional level of pressure unchanged, the shifting time practically changes due to various causes, such as changes in the frictional coefficients of frictional coupling elements and the deterioration in the viscosity of a working hydraulic oil in the hydraulic control circuit.
In an attempt to settle the difficulties, a learning control may be conducted in the transmission gear shift control system. For example, the automatic transmission control system described in the above-mentioned publication has a configuration in which, if an actual shifting time for a current gear shifting is different from a target shifting time, the level of line pressure is corrected so as to complete a next gear shifting within the target shifting time. In this instance, in order for the automatic transmission to prevent the prolongation of shifting time, the line pressure is corrected downward if the actual shifting time is shorter than the target shifting time and, on the other hand, is corrected upward if it is longer than the target shifting time.
However, with regard to this type of automatic transmission, the learning control of pressure level for locking or line pressure causes slippage of the frictional coupling element and a deteriorated response of the frictional coupling element.
To understand the drawback of the prior art automatic transmissions, reference is made to FIG. 13. In an automatic transmission control system which changes the transitional pressure level during a gear shift according to engine operating conditions, such as an engine throttle valve opening, the transitional pressure level is set approximately to an initial pressure level "a" as shown in FIG. 13. At this initial pressure level, an actuator for the frictional coupling element commences to operate so as to cause gentle locking of the frictional coupling element in the event that the engine throttle opening is small.
If a shifting time Tq, which is the time required to complete a gear shift after the gear shift is started, is shorter than a target shifting time, the learning control takes place so as to correctively change the line pressure downward for another gear shift. In this event, even if a pressure drop P in line pressure occurs as shown by line "b," what takes place in the other gear shift is only that a time Ts from the appearance of a shift command signal to the development of actuator pressure to the initial level is extended to a time Ts' as indicated by line "b." In other words, the gradient at which the transitional pressure level rises is not changed. Consequently, the shifting time Tq' for the other gear shift is approximately equal to the shifting time Tq for the previous gear shift. Unless there is caused a change in shifting time, the learning control lowers continuously the line pressure, resulting in allowing the frictional coupling element to slip and induce the deterioration of responsiveness.